I'm kicking around the idea of doing a tale from a D-Class perspective. It would follow the guy for the month as he's shunted from one job to the next. The arch would be that this guy is a stone cold killer forced to see and interact with horrible things, in the process reawakening his humanity. At the end he's begging for it to end. I don't want this to be flippant or crass. I want to take the time and make this…have a weight to it.

Has something like this been done before. And what would be some SCP's that he could interact with so as to get the desired result (NOT SCP-231!)

I'm not sure. It could work, but seems likely unless done extremely well to be highest-order narm. A tale from the viewpoint of a class-d on the other hand, seems to offer plenty of odd ideas. How about having him team up with a self-aware SCP so both of them can attempt an escape? 753 could work well for this. The class-d could have been assigned to bring it paints, and given a message via the finished paintings. The escape plan could be the class-d threatening to smash 753 if not let out, and 753 showing him the way out, which it remembers from when it was carried into its cell.